Thermobarometry of quartz diorite porphyry bodies and investigation of genesis of related skarn usingmineral chemistry data in Tighanab area,southeast of Sarbisheh (east of Iran)

In Tighanab area at southeast of Sarbisheh, the intrusion of quartzdioritic subvolcanic bodies into the Paleocene- Eocene limestone and sandstone led to the skarn formation. Plagioclase, hornblende (magnesiohornblende) and quartz are the main minerals in the subvolcanic rocks. Base on microprobe analyses, the composition of plagioclases with 15-45% anorthite content varies from andesine to oligoclase. The pressure during mineral crystallization of the Tighanab quartzdiorite porphyry is about 5.27kbar and its average temperature, on the basis of thermometry with coexisting hornblende and plagioclase, was obtained about788ºC. The Tighanab exo- skarn are characterized by the presence of garnet, epidote, pyroxene (diopside), calcite, chlorite, quartz, magnetite, pyrite, hematite and gypsum. The studied garnets belong to the calcic category and are of the andradite- grossular solid solution type. The composition of andradite varies slightly whereas the grossular constituents, to some extent, are uniform. The occurrence of andradite as well as its compositional changes represents high oxygen fugacity and Fe3+/Al ratio changes respectively, during the garnet crystallization. The presence of andradite, diopside- rich pyroxene and the over abundance of garnet relative to pyroxene in the studied skarn indicates oxidant conditions and correspond with the results of amphibole chemistry study in generating skarn quartz diorite porphyry. The lack of wollastonite in exo- skarn can be indication of garnet clinopyroxene crystallization at temperatures below 550 °C by metasomatizing solutions.